Currently, in modern illumination apparatuses, radiation sources which are energy-efficient, have high intensity and/or provide a high luminous power density are being used increasingly, such as high-power LEDs (light-emitting diodes), lasers, for example in the form of laser diodes, and/or superluminescent diodes. Unlike incandescent bulbs, which are thermal radiators, these radiation sources emit light in a narrowly limited spectral range, so that their light is almost monochromatic or exactly monochromatic. A possibility of developing further spectral ranges consists, for example, in radiation conversion, in which phosphors are irradiated by means of LEDs and/or laser diodes and in turn emit light of a different wavelength. In so-called remote phosphor applications, for example, a layer including phosphor located at a distance from a radiation source is conventionally illuminated by means of LEDs or laser diodes and in turn emits radiation of a different wavelength. For example, this technique may be used in order to convert light of blue LEDs by admixture of yellow light which is generated by excitation of a layer containing phosphor, into white light.
Furthermore, currently, projectors (beamers) are regularly used in order to represent data optically. Such a projector projects the data to be represented in the form of individual still and/or moving images, for example onto a canvas. In a conventional projector, it is known to generate the necessary excitation radiation with the aid of a conventional discharge lamp, for example an ultrahigh-pressure mercury vapor discharge lamp. Recently, however, LARP (Laser Activated Remote Phosphor) technology has also been used. In this technology, a conversion element which is arranged at a distance from the radiation source, and which includes phosphor or consists thereof, is irradiated with excitation radiation, in particular an excitation beam (pump beam, pump laser beam). The excitation radiation of the excitation beam is fully or partially absorbed by the phosphor and converted into conversion radiation (emission radiation), wavelengths and therefore spectral properties and/or color of which are determined by the conversion properties of the phosphor. In the case of down-conversion, the excitation radiation of the radiation source is converted by the irradiated phosphor into conversion radiation with wavelengths longer than those of the excitation radiation. For example, blue excitation radiation (blue laser light) can be converted into red or green conversion radiation (conversion light, illumination light) in this way with the aid of the conversion element.
The excitation radiation may introduce a large amount of energy into the conversion element, so that the latter may be heated strongly. This may cause damage to the conversion element and/or the phosphors contained therein, which may be present as a single phosphor or a phosphor mixture. Furthermore, in the event of insufficient cooling of the phosphor, conversion losses occur owing to efficiency reduction due to thermal quenching. In order to avoid excessive heating and therefore avoid the concomitant possible damage to the conversion element or the phosphor, it is known to arrange a plurality of conversion elements on a phosphor wheel (also referred to as a pump wheel or color wheel), which is irradiated with the excitation beam while it rotates. Owing to the rotation, different conversion elements and/or regions of the conversion element are illuminated successively, and the light energy introduced is therefore distributed over the surface.
A degree of miniaturization in LARP has so far been limited technology because of the design, since the arrangement which includes the radiation source (pump laser) and the phosphor wheel requires a great deal of installation space. For various applications, however, a small installation space is desirable, for example in the field of pico-projection, i.e. in the case of small-dimension mobile projectors, and/or miniaturized projection units for so-called embedded projection, in which the projection unit is integrated for example into a cell phone or a camera. Thermal connection of the conversion element is important in this case, in order to avoid overheating and damage.
For remote phosphor applications, thin phosphor layers such as cubic silicate minerals, orthosilicates, garnets or nitrides are applied onto surfaces of corresponding carriers. The phosphor layers are usually fixed mechanically by binders and connected to an optical system (lenses, collimators, etc.), in which case the light coupling may for example take place through air or an immersion medium. In order to ensure the best possible optical connection of the optical system to the phosphor, and in order to avoid light losses, optical connection which is as direct as possible should be ensured. For remote phosphor applications, that is to say applications in which the phosphor and the radiation source, for example high-power laser diodes, are spatially separated, a thin phosphor layer is for example applied onto a surface, for example of a substrate and/or of a carrier, mechanically fixed by binders and connected (air, immersion, etc.) to an optical system (lenses, collimators, etc.).
In the applications mentioned above, the phosphors are conventionally excited to emission by means of LEDs and/or laser diodes with high luminous powers. The heat losses then incurred are to be dissipated, for example via the carrier, in order to avoid overheating and therefore thermally induced changes in the optical properties, or even destruction of the phosphor. The phosphors are, for example, excited to emission by light sources with a high power density (a few W/mm2). The large (Stokes) heat losses then incurred lead to heat input in the phosphor layer. If these temperatures become too high, for example owing to insufficient cooling, thermally induced changes in the optical properties (emission wavelength, conversion efficiency, etc.) may occur, or ultimately destruction of the phosphors for the layer itself. The origin of this degeneration of the phosphor layer may be both the phosphor and the binder. For this reason, the phosphor layer should be configured in such a way that it can be cooled optimally in order to avoid thermal destruction of the phosphors and the binder.
In the absence of additional use of binders, for example silicones, the phosphors, which are usually present in powder form, do not form mechanically stable layers, that is to say abrasion- and/or scratch-resistant layers. Binders, moreover, are also generally used in order to bring the phosphor particles together to form a phase which can then be applied onto corresponding surfaces. When binders are used for layer stabilization, however, these binders themselves may interact with the phosphors and therefore negatively influence their optical and thermal properties, as well as their lifetime. Furthermore, the thermal conductivity of the binder often represents a limiting factor for the dissipation of heat incurred in the conversion element. Furthermore, the binder itself should be thermally and spectrally stable and exhibit little to no ageing properties. For this reason, the use of an inert, optically transparent and thermally and spectrally stable binder is advantageous for the production of stable phosphor layers with long lifetimes.
It is known to use silicones as binder matrices for phototechnological excitation (for example LEDs). These, however, do not allow very high luminous powers (power densities of a few W/mm2) or necessitate further technological outlay (for example color wheels to reduce the action time of the light). The known phosphor/silicone mixtures are usually applied directly on to metal substrates. For example, the phosphor is suspended in organic matrices, for example silicone, and then for example screen printed. The layers are, for example, about 30 μm thick. Silicone has a poor thermal conductivity (λsilicone=0.1-0.2 W/m·K), the effect of which is that the phosphor is heated more strongly during operation and therefore becomes less efficient. This is problematic particularly in the case of high-power LEDs and in laser applications.
The coating process when forming a phosphor layer is limited by the nature of the substrate materials. For instance, high-temperature processes cannot be envisioned on many plastics and metallic materials (for example aluminum) owing to their melting temperatures or thermal stability. Alternatively available highly thermally conductive ceramic materials (for example AlN), on the other hand, entail increased technological and financial outlay.
Inorganic matrices having improved heat dissipation are known from various documents, for example low-melting glass from WO 2011/104364 A1 or metal phosphates from WO 2011/138169 A1.
Inorganic matrices, however, have the disadvantage compared with organic matrices that relatively high temperatures are generally necessary in order to achieve a compact layer with no bubble content, when a certain chemical stability is required (for example in relation to UV radiation and/or moisture). Typical softening temperatures of common low-melting glasses are from 500° C. to 600° C. At these temperatures, optoelectronic substrates, for example an LED Chip, or highly reflective substrates, for example highly reflective aluminum, or the phosphor to be embedded, in particular nitrides, are already damaged and therefore become less efficient.
As alternatives, conversion elements which are formed from a ceramic including the phosphor or from a crystal including the phosphor are known. In particular, the phosphor may form the ceramic or the crystal. Such conversion elements can be adhesively bonded firmly to heat sinks, so that the heat incurred therein can be dissipated. A limiting factor for the dissipation of the heat is in this case the thermal conductivity of the adhesive used. Furthermore, good heat dissipation is beneficial when the conversion elements are formed particularly thinly.